Reactivity controlled compression ignition (RCCI) utilizes two fuels with different reactivities and multiple fuel injections to control air-fuel mixture reactivity in engine cylinders. Further, one of the fuels has a lower reactivity as compared to the other fuel. A homogenous charge comprising the low reactivity fuel, exhaust gas recirculation (EGR), and air is formed in an engine cylinder. As a piston in the cylinder compresses the homogenous mixture, the higher reactivity fuel may be injected at a time before ignition of the homogenous mixture is ignited. Combustion of the air-fuel mixture may be controlled in this way to reduce NOx and particulate matter while improving engine fuel economy. However, changes in the in cylinder oxygen concentration and/or boost related to transient engine operating conditions can affect the composition of cylinder charge mixture and degrade combustion.
The inventor herein has recognized the above-mentioned disadvantages and has developed an engine method, comprising: increasing an amount of a first fuel with a first reactivity injected to a cylinder during a transition greater than a threshold in response to an engine intake oxygen concentration error; decreasing an amount of a second fuel with a second reactivity injected to the cylinder during the transition in response to the engine intake oxygen concentration error; and compression igniting the first and second fuels in the cylinder.
By adjusting amounts of fuels having different reactivity rates injected to a cylinder during transient conditions in response to an engine air intake oxygen concentration error, it may be possible to improve transient engine emissions while at the same time providing a desired engine torque amount. For example, an engine air intake oxygen concentration can be controlled during steady state operating conditions so that NOx and particulate matter may be reduced. However, during transient conditions, the engine air intake oxygen concentration may be in error as compared to the desired engine air intake oxygen concentration. The engine air intake oxygen concentration error may be compensated via increasing an amount of a first fuel with a first reactivity injected into the cylinder during a cylinder cycle and decreasing an amount of a second fuel injected into the cylinder during the cylinder cycle. In this way, engine emissions during transient engine operating conditions may be controlled to approach steady state engine emissions during similar engine operating conditions
The present description may provide several advantages. In particular, the approach may reduce engine emissions when fuels with different levels of reactivity are combusted in an engine. Further, the approach may allow for reduced sizing of exhaust after treatment devices since transient engine emissions may be improved. Further still, the approach may improve vehicle drivability during some conditions by providing a desired level of engine torque rather than misfiring or hesitating.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.